Acting in an overly fake or exaggerated way; or the process of making cheese; also grinning widely (slang from 'cheese' for camera smile).
From 'cheese' + gerund '-ing'. Multiple meanings: the legitimate cheese-making process, plus slang 'cheese for camera' (to smile wide), plus theatrical 'being cheesy.' The camera smile meaning came from photography.
When photographers asked people to say 'cheese' to make them smile, they created one of language's silliest traditions—it works because 'cheese' forces your lips into smile shape while you say it!
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